1. Field of the Invention
The present invention relates to a belt-conveyance control device, an image forming apparatus, a belt-conveyance control method, and a computer program product for controlling belt conveyance.
2. Description of the Related Art
A typical image forming apparatus includes a belt conveying device such as a transfer belt, a photosensitive belt, or a sheet conveying belt. Driving such belts at a constant speed is necessary for obtaining images with high quality. For example, technologies are disclosed in Japanese Patent Application Laid-open No. S62-242965 and Japanese Patent Application Laid-open No. 2004-187413 to achieve a constant speed driving of the belt.
The technology disclosed in Japanese Patent Application Laid-open No. S62-242965 is to drive a belt that is supported by a drive shaft and a driven shaft at a constant speed by attaching an encoder for measurement to a driven roller and performing a feedback control based on information output from the encoder. Moreover, the technology disclosed in Japanese Patent Application Laid-open No. 2004-187413 is aimed at obtaining a drive control system with high accuracy, specially, by using a pulse motor. The belt conveying device in Japanese Patent Application Laid-open No. 2004-187413 includes a belt supported by a drive shaft and a driven shaft, and a power transmission system such as a pulse motor to drive the drive shaft to rotate, in which the drive shaft is driven to rotate by inputting driving pulses to the pulse motor thereby driving the belt at a constant speed. Specifically, in the belt conveying device, an angular displacement detecting unit is provided to the driven shaft, and a feedback system is formed to calculate a motor driving correction pulse frequency based on the difference between a target angular displacement of the driven shaft and a detected angular displacement, and the pulse motor is driven with a frequency that is obtained by adding the motor driving correction pulse frequency to a reference driving pulse frequency.
Both of the above technologies can cause the belt to drive at a constant speed. However, if the driven roller to which the encoder is attached is deflected, fluctuation component of the driven roller in one rotation occurs depending upon the amount of deflection of the driven roller.
A technology to solve the above problem is disclosed, for example, in Japanese Patent Application Laid-open No. 2000-047547, in which an outer diameter of a driven roller is an integral multiple or an inverse of an integral multiple of that of the drive roller to separate rotation frequencies of both rollers, and only the deflection component of the driven roller is canceled.
Furthermore, a technology disclosed in Japanese Patent Application Laid-open No. 2001-66909 is also known, in which a belt is supported by a driven roller and a drive roller, an encoder is attached to the driven roller to detect fluctuation of the drive roller in one rotation, and the belt is controlled by open control.
However, in the technology disclosed in Japanese Patent Application Laid-open No. 2000-047547, there is a limitation in layout of the driven roller and a drive roller, so that apparatuses capable of employing this technology are limited. In the technology disclosed in Japanese Patent Application Laid-open No. 2001-66909, a correction value is measured by the encoder attached to the driven roller, however, there is no description about deflection of the encoder. Moreover, because the feedback control is not performed, fluctuation of other components cannot be controlled. Therefore, driving the belt with high accuracy is difficult as a whole.